Embodiments of the present invention relate to spread spectrum solutions for electromagnetic interference (EMI) in switched mode power supplies by utilization of spread spectrum switching frequencies.
Switched mode power supplies, due to the nature of their switching behavior, introduce spectral spurs at their fundamental switching frequency and corresponding harmonics. These spurs are referred to as electromagnetic interference (EMI) and are regulated by the CISPR, FCC and other standards. FIG. 1B illustrates an EMI spur for a 2.2 MHz fundamental frequency at −24.2 dB. This is for a square wave with no spread spectrum switching. FIG. 1A illustrates a corresponding low frequency spur at −79.7 dB associated with harmonics of the 2.2 MHz fundamental frequency. Board level solutions to such interference typically utilize a combination of shielding or filter techniques to suppress EMI spurs in order to comply with regulations and design specifications. However, board level methods to mitigate EMI through layout techniques fail to address the source of noise generation. Best practice layout techniques can only mitigate the introduction of additional EMI noise by minimizing current conducting loop area, filtering, shielding, and use of ground planes. Furthermore, these methods increase system cost as well as solution size.
Spread spectrum switching is a control technique to dither or change the switching frequency over a predetermined bandwidth. This reduces the EMI spur at the fundamental frequency by spreading the spectral energy over adjacent frequencies. There are two broad categories for spread spectrum algorithms. In the first category of fixed pattern dither algorithms, Apps Team, “A Solution for Peak EMI Reduction with Spread Spectrum Clock Generators,” ON Semiconductor Application Note AND9015, (July 2011) disclose triangular (FIG. 1) and Hershey Kiss (FIG. 2) spread spectrum profiles. Kumar et al., “Reducing EMI in Digital Systems Through Spread Spectrum Clock Generators,” Cypress Semiconductor Application Note published in EE Times Design, 1, 16 (February 2011) also compare triangular (FIG. 5a) and Hershey Kiss (FIG. 5b) spread spectrum profiles. Hardin et al., U.S. Pat. No. 5,488,627 discuss various fixed pattern, spread spectrum profiles. Details of the foregoing references are incorporated by reference herein in their entirety. Fixed pattern dither algorithms provide the best reduction of fundamental frequency spurs at the cost of introducing large spurs at the modulation frequency of their fixed patterns. This additional spectral noise is further exacerbated when optimizing for the CISPR/FCC specifications and results in modulation spurs being placed in the audio band around 9 kHz. This may cause an undesirable hum in switching power supplies operating in the MHz range.
FIG. 3B illustrates the spectral energy of a fixed pattern, triangular modulation curve of the prior art with a 2.2 MHz center frequency. The spectral energy is spread between 2.0 MHz and 2.4 MHz with a maximum of −36.6 dB. FIG. 3A illustrates a corresponding low frequency spectrum having a dominant EMI spur of −76.6 dB at 9.2 kHz. By way of comparison, FIG. 4B illustrates the spectral energy of a fixed pattern, Hershey Kiss modulation curve of the prior art with a 2.2 MHz center frequency. The spectral energy is spread between 2.0 MHz and 2.4 MHz with a maximum of −29.2 dB. FIG. 4A illustrates a corresponding low frequency spectrum having a dominant EMI spur of −77.9 dB at 1.0 kHz. Both triangular and Hershey Kiss modulation curves reduce EMI with spread spectrum switching. However, both produce corresponding low frequency EMI spurs in the audio band due to their respective modulation frequencies.
In the second category of spread spectrum algorithms, Lin et al., “Reduction of Power Supply EMI Emission by Switching Frequency Modulation,” IEEE Trans. on Power Electronics, Vol. 9, No. 1, 132, 137 (January 1994) disclose a pseudorandom dither algorithm of spread spectrum switching. Details of the foregoing reference are incorporated by reference herein in their entirety. Pseudorandom variation of the fundamental frequency, however, provides inferior fundamental spur reduction but decreases other spectral content. This is illustrated at FIG. 2B where fundamental frequency spectral energy is spread between 1.8 MHz and 2.6 MHz. A large spur of −27.3 dB still exists at the 2.2 MHz center frequency. However, corresponding low frequency spurs of FIG. 2A have a maximum noise floor of −84.9 dB.
The foregoing spread spectrum algorithms reduce EMI at the source through spread spectrum techniques. However, the present inventors have realized a need to further reduce EMI in switching power supplies. Accordingly, the preferred embodiments described below are directed toward improving upon the prior art.